A fibronectin type III (Fn3) polypeptide monobody, a nucleic acid molecule encoding a monobody, and a variegated nucleic acid library encoding a monobody, are provided by the invention. Also provided are methods of preparing an Fn3 polypeptide monobody, and kits to perform such methods. Further provided is a method of identifying the amino acid sequence of a polypeptide molecule capable of binding to a specific binding partner (SBP) so as to form a polypeptide:SSP complex, and a method of identifying the amino acid sequence of a polypeptide molecule capable of catalyzing a chemical reaction with a catalyzed rate constant, k.sub.cat, and an uncatalyzed rate constant, k.sub.uncat, such that the ratio of k.sub.cat/k.sub.uncat is greater than 10.